Numerical modelling of a transient slug mixing experiment of the ROCOM test facility using CFX-5

The coolant mixing is of relevance for two classes of accident scenarios – boron dilution and cold-water transients. Experimental investigations and numerical simulations of coolant mixing in the downcomer and the lower plenum of PWRs was the topic of the EU-Project “FLOMIX-R”. The results of the project showed, that coolant mixing has a high relevance for nuclear safety. The coolant mixing is the only inherent mechanism, which reduces risks of power excursions. For the investigation of the relevant mixing phenomena, the Rossendorf test facility ROCOM has been used. ROCOM is a 1:5 scaled Plexiglas model of the PWR Konvoi allowing conductivity measurements by wire mesh sensors and velocity measurements by the LDA technique. Series of experiments were performed at ROCOM to study the mixing of a slug of lower borated water during the start-up of the first main circulation pump.

The CFD calculations were carried out with the code CFX-5. A complex hybrid mesh with the size of 2 million nodes and 4 million elements was used. The turbulence was modeled with the SST model.

Due to the strong momentum insertion during the pump start-up, a dominating horizontal component of the flow was observed near the inlet nozzle leading to a circumferential flow around the core barrel. Therefore, the injected slug is distributed into two main jets, and the maximum of the tracer concentration at the core inlet appears firstly at the location opposite to the loop where the tracer was injected. For turbulent flows the CFD-Code CFX-5 was validated for turbulent, momentum driven mixing. A better description of the mixing processes inside the RPV is the basis of a more realistic safety assessment concerning boron dilution scenarios.